A biconical antenna, as well as other similar tapered dipole and monopole antennas, including bowtie or Brown-Woodward dipoles and discones, can provide a very broad impedance bandwidth. However, this performance does not extend down into the range in which the antenna is electrically-small. For example, a biconical antenna with a flare angle of 120 degrees can be matched using a 4:1 balun to provide better than 2:1 VSWR over a 6:1 bandwidth. However, the antenna is about one-half wavelength wide at the lower end of this operating band. Thus, as the frequency of interest drops below the operating band, the relative electrical size of the antenna becomes small when compared with the wavelength, decreasing the efficiency of the antenna significantly.
The biconical antenna is of particular interest in applications such as testing noise immunity and electromagnetic emissions. To ensure that the results of such tests are repeatable and can be compared with the results of other tests using different biconical antennas, various well accepted standard antenna specifications have been developed. Once such standard biconical antenna design, defined by U.S. Military Standard 461A (Aug. 1, 1968) is illustrated in FIG. 1.
As depicted in FIG. 1, a conventional biconical antenna 10 used in the EMC industry comprises two outrigger assemblies 12 which are skeletal approximations of a conic surface. The outrigger assemblies 12 are connected to a matching balun 14 by an appropriate coupling 16. The outrigger assemblies are formed of ribs 13 connected between the coupling 16 and endpoint 17 of a central support 18. The balun 14 is used to transfer received and transmitted energy between the antenna 10 and a suitable transmitter and/or receiver, respectively. The antenna 10 is about 1.37 meters in width and has a flare angle of 30 degrees.
For biconical antennas of this type, it is generally expected that good performance can be obtained for frequencies above 100 MHz and, in fact, most commercially available biconical antennas complying with MIL-STD-461A provide excellent performance from 100 MHz to 300 MHz. Acceptable performance can often extends to 60 MHz. However users often attempt to use the biconical antenna at frequencies down to 26 MHz. Unfortunately, these biconical antennas are notorious for poor performance in the 30-60 MHz range. In fact, at 30 MHz, the input match for these commercial antennas is so poor that input VSWR is actually determined primarily by line and balun losses. The poor input match results in extremely high "mismatch loss" and thus severely reduces gain.
Thus, the ability of the traditional 1.37 meter biconical antenna to generate electric field (for immunity testing) with a given input power is very poor. A further consequence of the extreme mismatch is the high voltage at the input connector generated by the near doubling of the input voltage over that which would exist on a matched line with the same forward power. This doubling of the input voltage stresses connectors to the point that they often fail from electric field breakdown.
Despite poor low frequency performance, the biconical antenna has attained universal acceptance in the EMC industry. The design of the 1.37 meter biconical antenna is rooted firmly in MIL-STD 461. Its design is very much standardized and biconical antennas from any of the leading EMC test equipment manufacturers perform almost identically. This ensures that repeatable measurements can be obtained without regard for the antenna manufacturer. In addition, the standard biconical antenna design provides a mechanically robust easily-transported, and rapidly-assembled device. Because of this, users of biconical antennas are reluctant to adopt any designs which depart drastically from the standard.
Various techniques have been proposed to improve the performance of biconical antennas in the low frequency range. In one technique, an impedance matching network is incorporated into the BALUN enclosure to improve the input VSWR for the biconical antenna over the 30-60 MHz range. Because the network is incorporated into the BALUN, no changes to the external geometry of the antenna are required. However, the improvement provided by such a network is generally quite small because no amount of input impedance matching can change the instrinsically high radiation Q of the biconical antenna in the frequency range in which it is electrically-small. In other words, while the biconical geometry provides excellent performance over a frequency range in which it is of moderate electrical size, is simply not a good electrically-small antenna.
Therefore, instead of using a modified biconical antenna, many user rely on a second alternate antenna for work in low frequency ranges. A popular alternate antenna is the top or end loaded dipole. Top loading provides improved performance at low frequencies by increasing the shunt capacity of the antenna, thus lowering the fundamental resonance frequency, and by providing a charge reservoir at the end of the antenna, increasing the current density near the outer ends of the antenna.
Top loaded dipole antennas can be reliably designed to cover the 30-100 MHz range. Unfortunately, the top loaded dipole antenna does not provide good performance over the frequency range in which it is of moderate electrical size. A top-loaded dipole (with 1.37 meter width) antenna provides good performance over the 30-60 MHz range and acceptable performance up to 100 MHz. This is a frequency range which is nearly disjoint, but also nearly complementary, to the 100-300 MHz operating range of the 1.37 meter biconical antenna.
However, while two antennas are sufficient to adequately cover testing from 30 MHz to 300 MHz, their use requires that operators purchase, transport, and store two relatively large antennas. In addition, it is often desired to rapidly make measurements throughout the 30 MHz to 300 MHz range. Unfortunately, decoupling one antenna from the measuring device, removing it from the testing area of interest, and replacing it with the alternate antenna can be cumbersome and time consuming.
Accordingly, it is an object of the present invention to provide a biconical antenna which has good performance over the 100-300 MHz range of conventional antenna designs, while also achieving good performance over the 30-60 MHz range.
It is a further object of the invention to provide a biconical antenna which complies with accepted biconical antenna design standards to provide for repeatable measurements while also being easily and reversibly modified for improved performance at low frequency ranges.